Mammalian flavin-containing monooxygenase (FMO) metabolizes a great number of therapeutic and illegal drugs and plant alkaloids including tamoxifen, codeine and nicotine. Unlike the cytochrome P450 (CYP) monooxygenases, FMO usually produces inactive, non-toxic metabolites. The FMO superfamily has 5 families, each with a single member. FMO2, the major form in lung, is developmentally regulated, appearing during late gestation in fetal lung and reaching adult levels soon after birth, suggesting a critical function in protection of the fetus and neonate against foreign chemicals. This study explores the structural basis for the properties of FMOs in general, and FMO1 and FMO2 in particular utilizing chimeras and site-directed mutagenesis. Expression of Rhesus and human FMO2 cDNA will make possible, for the first time, study of the properties of the primate orthologs. Humans appear unique in transcription of a non-functional mRNA. Collaborative studies with the University of London will investigate an apparent polymorphism, evident in individuals of African descent, heterozygous for a full-length mutant. In vitro studies of FMO regulation will be performed utilizing high-precision lung slices. FMOs are present in high concentration in nasal tissue. The contribution of nasal FMO (relative to CYP) in the metabolism of nicotine will be determined, as will the potential for modulation of nasal FMO activity in vitro and in vivo. Indole-3-carbinol (I3C), a major component of cruciferous vegetables currently being examined in human clinical trials, represses FMO activity and FMO1 protein levels in liver and intestine, while inducing a number of CYPs, providing a model for the contribution of FMO in drug metabolism in vivo. Results from these studies will further the understanding of the importance of this enzyme in human health.